1. Field of the Invention
The Invention relates to rotary drag-type drill bits for use in drilling holes in subsurface formations. In particular, the invention is a method for determining operating characteristics of a rotary drag-type drill bit due to forces acting on its cutting elements.
2. Description of Related Art
The invention is particularly, but not exclusively, applicable to drill bits in which some or all of the cutters are preform cutters formed, at least in part, from polycrystalline diamond or other superhard material. One common form of cutter comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the cutter, bonded to a substrate which is usually of cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be molded using a powder metallurgy process in which tungsten carbide power is infiltrated with a metal alloy binder in a furnace so as to form a hard matrix.
The cutters on the drill bit have cutting edges which, together, define an overall cutting profile which defines the surface shape of the bottom of the borehole which the bit drills. Preferably the cutting profile is substantially continuous over the leading face of the bit so as to form a comparatively smooth bottom hole profile.
The contribution which an individual cutter makes to the cutting action of the bit, and, in particular, to the forces acting on the bit, is subject to a number of variables. For example, such factors will vary according to the axial and radial position of each cutter relative to the other cutters. Thus, if a cutting element is radially located on the bit so that its path of movement partly overlaps the path of movement of a preceding cutter, as the bit rotates, it will be subject to lower forces than would be the case if it were radially positioned so that such overlapping did not occur, or occurred to a lesser extent, since the leading cutter will already have removed some material from the path swept by the following cutter.
Similarly, a cutter which is axially positioned so that it projects further than another similar cutter from the surface of the bit body may be subject to higher forces than said cutter. In practice the action of each cutter may be affected by the action of a number of other cutters which are at adjacent relative radial and axial positions. It will be appreciated that such cutters will not necessarily be directly adjacent one another on the actual bit body but may well be angularly displaced circumferentially from one another by a considerable distance.
In order to determine the forces acting on a particular drill bit in use, such as the effect of the cutters on weight-on-bit, torque, and any out of balance force and out of balance angle for the bit, it is desirable to be able to make an analysis of the contribution to such forces by individual cutters. This enables the force characteristics of a particular bit design to be determined and the effect of modification of the design, for example by re-positioning cutters, to be studied.
It is common practice to use computers to model and analyze bit designs and various methods of analysis have been proposed. It will be appreciated that such analysis may conveniently be carried out by constructing a computerized model or representation of a particular bit design, certain operating characteristics of the bit then being determined or estimated by a computer program which performs a series of steps on the computerized model of the bit.
The present invention sets out to provide a novel and improved method of determining characteristics of a drill bit design, and particularly for estimating the effect of cutter placement on the forces acting on the bit in use.
The method will be defined by a series of analytical steps and, for convenience and to assist understanding, such steps will be described as if being applied to physical elements. However, it will be appreciated that in practice such methods lend themselves to performance using a computer and the described steps will normally in practice be embodied in a computer program.